1. Field of the Invention
This invention relates to proppants, and more specifically, the use of proppants in hydraulic fracturing of subterranean formations.
2. Description of Related Art
Hydraulic fracturing is a process used to stimulate oil and gas production in subterranean formations. A fracturing fluid is injected into the well casing or tubing to produce a buildup of well bore pressure. When the well bore pressure is large enough to overcome compressive earth forces, fractures form. Continued injection of the fracturing fluid increases fracture length and width.
The fracturing fluid typically includes a viscous fluid, such as, for example, a linear gel or a crosslinking gel, that carries hydraulic fracturing particles commonly known as proppants. Once the fracturing fluid transports the proppant inside the fracture, the viscosity of the fracturing fluid breaks down leaving the proppant particles in place. The fracturing fluid is then dissipated through the formation or recovered via the well bore.
Proppant particles are commonly made of sand, glass, bauxite, ceramic, and shells and range in size from about 6 to 200 mesh (U.S. Sieve Series scale). Many of these proppant particles are optionally supplied with a resin coating. A resin-coated proppant particle generally has a greater crush resistance than the core substrate material and can be used at greater depths or may be used at the same depth with increased relative conductivity. For example, resin-coated sand can be used at closure stresses up to about 10,000 psi, as compared with about 6,000 psi. for uncoated sand. The coating also serves to trap free fines from fragmented or disintegrated substrates under high closure stress.
Compressive earth stresses, or closure stresses, encountered in subterranean formations subjected to fracturing can range from 500 psi to 25,000 psi or even higher, depending upon the depth of the fracture. Accordingly, the proppant selected for a particular subterranean formation must be strong enough to resist the compressive earth forces in that formation, thereby keeping the fractures open and allowing fluid flow therethrough. Thus, sand proppant particles are generally used where closure stresses are up to about 6,000 psi, ceramics are generally used at closure stresses up to about 15,000 psi, and bauxite is generally used at closure stresses greater than about 15,000 psi.
The productivity of a subterranean fracture is dependent upon, among other things, the conductivity of the fracture. Conductivity is defined as the permeability of the proppant times the width of the fracture. Permeability, in turn, refers to the permeability of a mass or "pack" of the proppant particles and is governed by the size and shape of the proppant particles and the size of the interstitial spaces between the particles. These interstitial spaces become the passageways for the flow and recovery of subterranean fluids. Accordingly, a proppant consisting of coarse proppant particles can form a proppant pack having larger interstitial spaces to achieve higher conductivity relative to a proppant consisting of finer proppant particles.
Despite the advent of modern proppants, conductivity of proppant-filled fractures continues to be a problem. In formations at shallower depths where closure pressures are typically about 4000 psi or less, coarser proppant particles greater than about 40 mesh are commonly used. However, coarser proppant particles are susceptible to bridging in fractures during stimulation treatment. Bridging occurs when the coarse proppant particles plug narrow fractures. The result is a reduction in the overall productivity of the well bore or premature screen-out. Moreover, the stress encountered even in shallower formations may produce fines that migrate and plug the interstitial flow passages between the proppant particles, thereby drastically reducing the conductivity of the fracture.
In fractures located at greater depths where closure pressures are higher, e.g. 10,000 psi or more, proppants composed of finer particles are typically used. However, because the interstitial spaces between these finer particles is smaller, permeability and hence conductivity are inherently less.
It is desirable to have a method of increasing the conductivity of a subterranean fracture. It is also desirable to develop a proppant having increased permeability for a given particle size. It is also desirable to replace coarse proppant particles with finer proppant particles in subterranean fracturing operations while maintaining or improving the conductivity of the fracture.